This invention relates to a magnet assembly and more particularly relates to a combination of a magnet with a high permeability pole piece mounted at one pole of the magnet to render the magnetic field there more uniform, and to such an assembly further including a dual differential magnetic-field to electrical-signal transducer mounted to the pole piece for detecting the passage of ferroelectric objects.
In the past, high permeability pole pieces of ferrite, iron low carbon steel and the like have been used in conjunction with a magnet to direct and/or concentrate the magnetic field of the magnet in a desired region. The uniformity of the magnetic field at a region near the poles of a magnet has normally been considered quite uniform, at least uniform enough for the uses to which the magnet has to be put.
Proximity sensors are extensively used for detecting the approach of a ferromagnetic body. Many such sensors are comprised of a magnetic field detector mounted at the pole of a magnet. Such a device is described in the patent to G. Avery, US 4,443,716 issued Apr. 17, 1984. In that device the integrated circuit includes a Hall element followed by a differential voltage comparator or Schmitt trigger circuit.
There is also described in another patent to Avery, US 4,518,918 issued May 21, 1985, a magnet having mounted at one pole two integrated circuits each being of the kind including a Hall element followed by an amplifier. The outputs of the two amplifiers are so combined so as to obtain a voltage related to the difference.
However, it has been discovered that the lack of uniformity of the magnet-field at a magnet pole end leads to difficulty in positioning two magnetic sensors there so that in the absence of another ferroelectric object in the vicinity, the magnetic field strength at one sensor element is equal to that at the other.
It is therefore an object of this invention to provide a magnet assembly having a broad region at one pole end that is uniform.
It is a further object of this invention to provide such an assembly having a dual Hall sensor at the one pole producing a sensor difference voltage proportional to the gradient field at that pole that may be attributable to a passing ferromagnetic object.